1. Field of the Invention
The present invention relates to a charge roller rotatable in contact with a photoconductive element in the form of a drum or a belt and applied with a voltage for charging the element, and an image forming apparatus using the same.
2. Discussion of the Background
In an electrophotographic image forming apparatus, a corona discharger is a predominant implementation for uniformly charging the surface of an image carrier implemented as a photoconductive element. With a corona discharger, it is possible to charge a photoconductive element to a predetermined potential. However, the problem with a corona discharger is that it needs a high-tension power source and produces ozone when caused to discharge. A great amount of ozone not only pollutes the environment but also causes a charging member and a photoconductive element to deteriorate.
A charge roller is another conventional scheme for uniformly charging the surface of a photoconductive drum. A charge roller is rotatable in contact with a photoconductive drum and is applied with a voltage. A state-of-the-art charge roller, however, is inferior to the corona discharger in respect of uniform charging, although it is operable with a low voltage source and reduces ozone.
To promote uniform charging with a charge roller, an AC voltage whose peak-to-peak voltage is twice as high as a charge start voltage (V.sub.TH) when a DC voltage is applied may be superposed on the DC voltage, as taught in Japanese Patent Laid-Open Publication No. 63-149668. However, such an AC-biased DC scheme needs an AC power source in addition to a DC power source, increasing the overall cost of the image forming apparatus. Moreover, an AC current, used in a great amount, produces a great amount of ozone, again resulting in the deterioration and environmental pollution problems.
When only a DC voltage is applied to a charge roller, charging becomes irregular since the roller has an elastic layer implemented by a dispersion of synthetic rubber and carbon. Specifically, irregular charging particular to a conventional charge roller stems from electrical irregularity in the elastic carbon/synthetic rubber layer thereof. The irregularity can be eliminated if the elastic layer is implemented by epichlorohydrin rubber having medium resistance. While another issue with the application of only a DC voltage is the withstanding voltage of the roller layer, experiments showed that an elastic layer made of epichlorohydrin rubber noticeably increases the withstanding voltage, compared to the conventional carbon/synthetic rubber mixture. In addition, epichlorohydrin rubber has rubber hardness as high as 40 (prescribed by JIS A) and undergoes hardly any elastic deformation and,therefore, has mechanical strength great enough to enhance the durability of a charge roller.
The AC-biased DC voltage uniformly charges a charge roller due to the superposition of AC even when potentials having formed a latent image or some toner remains on a photoconductive element. However, uniform charging is not attainable with a DC voltage alone unless a charging step begins at the surface of a photoconductive element preliminarily exposed by a quenching lamp, and the charging step begins at the surface of the element from which the residual toner has been fully removed by a cleaning blade, not to speak of unless the charge roller is made of a non-dispersed epichlorohydrin-based elastic substance.
With the conventional carbon/synthetic rubber layer, it is extremely difficult to achieve both of an adequate conductivity (medium resistance) and an adequate withstanding voltage, although such a layer allows the apparent electric resistance to be adjusted on the basis of the content of carbon. Moreover, this kind of elastic layer, in a microscopic view, has electric resistance noticeably differing from carbon portions to synthetic rubber portion, resulting in non-uniform charging and low withstanding voltage. In contrast, when the elastic layer is implemented by epichlorohydrin rubber, which itself has medium resistance (10.sup.7 .OMEGA.cm to 10.sup.10 .OMEGA.cm), not containing carbon or similar conductive particles, the problems attributable to the electric characteristic of the charge roller can be eliminated.
However, even though a charge roller may have desirable electrical and mechanical characteristics stated above and can uniformly charge a photoconductive element only with a DC voltage, it lacks sufficient durability as a charger when incorporated in a particular kind of copier, e.g., high speed copier. When a photoconductive element built in a high speed copier fails to have the surface thereof fully cleaned, a small amount of toner remains on the element even after a cleaning step. This part of toner is transferred to the charge roller which is rotating in contact with the photoconductive element, thereby degrading the charging ability of the charge roller.
While some different approaches have been proposed for cleaning a charge roller, in practice it is extremely difficult to remove a toner and other impurities automatically and completely from the surface of a charge roller. Therefore, with a cleaning device only, it is impracticable to fully clean the surface of a highly durable charge roller, i.e., to maintain the charging ability of the roller constant.